bug#37812: Update Emacs manual intro with more compelling features

[Nicholas Drozd <nicholasdrozd@gmail.com>]

The introduction to the Emacs manual says, right near the beginning:
"We call Emacs 'advanced' because it can do much more than simple
insertion and deletion of text.  It can control subprocesses, indent
programs automatically, show multiple files at once, and more."

This text has been around for a long time*. Maybe it's time for an
update? Indenting programs automatically and showing multiple files at
once aren't very compelling features these days. My go-to example of a
remarkable Emacs-y feature is EWW, so maybe "browse the Web" could be
added? Or something to do with Org mode? I don't know exactly, but
listing "shows multiple files" as a feature is like describing someone
as "punctual" in a reference letter -- it could signal that the person
has no other important qualities.

* How old exactly, I'm not sure. It appears in the 18.59 manual from
1992 (ftp://ftp.gnu.org/old-gnu/emacs/), so I assume it goes back even
further than that. The passage underwent a minor facelift in 2006
(commit 350287ef), but besides that it has gone substantially
unchanged.